Fix matching of second-stage HTLC claim in get_htlc_balance

We incorrectly assumed that the descriptor's output index from
second-stage HTLC transaction would always match the HTLC's output index
in the commitment transaction. This doesn't make any sense though, we
need to make sure we map the descriptor to it's corresponding HTLC in
the commitment. Instead, we check that the transaction from which the
descriptor originated from spends the HTLC in question.

Note that pre-anchors, second-stage HTLC transactions are always 1
input-1 output, so previously we would only match if the HTLC was the
first output in the commitment transaction. Post-anchors, they are
malleable, so  we can aggregate multiple HTLC claims into a single
transaction making this even more likely to happen. Unfortunately, we
lacked proper coverage in this area so the bug went unnoticed. To
address this, we aim to extend our existing coverage of
`get_claimable_balances` to anchor outputs channels in the following
commits.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -1751,7 +1751,19 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 				},
 				OnchainEvent::MaturingOutput {
 					descriptor: SpendableOutputDescriptor::DelayedPaymentOutput(ref descriptor) }
-				if descriptor.outpoint.index as u32 == htlc_commitment_tx_output_idx => {
+				if event.transaction.as_ref().map(|tx| tx.input.iter().enumerate()
+					.any(|(input_idx, inp)|
+						 Some(inp.previous_output.txid) == confirmed_txid &&
+							inp.previous_output.vout == htlc_commitment_tx_output_idx &&
+								// A maturing output for an HTLC claim will always be at the same
+								// index as the HTLC input. This is true pre-anchors, as there's
+								// only 1 input and 1 output. This is also true post-anchors,
+								// because we have a SIGHASH_SINGLE|ANYONECANPAY signature from our
+								// channel counterparty.
+								descriptor.outpoint.index as usize == input_idx
+					))
+					.unwrap_or(false)
+				=> {
 					debug_assert!(holder_delayed_output_pending.is_none());
 					holder_delayed_output_pending = Some(event.confirmation_threshold());
 				},
@@ -1892,8 +1904,7 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitor<Signer> {
 	/// confirmations on the claim transaction.
 	///
 	/// Note that for `ChannelMonitors` which track a channel which went on-chain with versions of
-	/// LDK prior to 0.0.111, balances may not be fully captured if our counterparty broadcasted
-	/// a revoked state.
+	/// LDK prior to 0.0.111, not all or excess balances may be included.
 	///
 	/// See [`Balance`] for additional details on the types of claimable balances which
 	/// may be returned here and their meanings.